1. Field of the Invention
The present invention relates to a capacitor. More particularly, the present invention relates to a capacitor comprising a ceramic dielectric and at least two electrodes, said dielectric being predominantly composed of a dielectric ceramic composition whose main component comprises alumina and which further comprises barium titanium oxide, magnesium oxide, silicon oxide, zirconium oxide and hafnium oxide.
As is known a capacitor is an electric circuit element used to store charge temporarily, consisting in general of two metallic plates separated and insulated from each other by a dielectric. Thus a capacitor is most simply defined as two conductors separated by a dielectric.
More specifically, a capacitor is a passive electronic component that stores energy in the form of an electrostatic field. In its simplest form, a capacitor consists of two conducting plates separated by an insulating material called the dielectric, The capacitance is directly proportional to the surface areas of the plates, and is inversely proportional to the separation between the plates. Capacitance also depends on the dielectric constant of the substance separating the plates.
The standard unit of capacitance is the farad, abbreviated F. This is a large unit; more common units are the microfarad, abbreviated μF(1 μF=10−6 F) and the picofarad, abbreviated pF (1 pF=10=12 F).
Capacitors can be fabricated onto integrated circuit (IC) chips. They are commonly used in conjunction with transistors in dynamic random access memory (DRAM). The capacitors help maintain the contents of memory. Because of their tiny physical size, these components have low capacitance.
They must be recharged thousands of times per second or the DRAM will lose its data.
Capacitors prepared according to the present invention are not only suitable for integrated circuit chips but are in fact suitable for all known capacitor uses.
Large capacitors are used in the power supplies of electronic equipment of all types, including computers and their peripherals. In these systems, the capacitors smooth out the rectified utility AC, providing pure, battery-like DC.
A dielectric is a material that is a good insulator (incapable of passing electrical current), but is capable of passing electrical fields of force.
Thus as is known, a good dielectric should also have other properties. It must resist breakdown under high voltages; it should not itself draw appreciable power from the circuit, it must have reasonable physical stability; and none of its characteristics should vary much over a fairly wide temperature range. One important application of dielectrics is as the material separating the plates of a capacitor. A capacitor with plates of a given area will vary in its ability to store electric charge depending on the material separating the plates. On the basis of this variation each insulating material can be assigned a dielectric constant. Generally, the dielectric constant of air is defined as 1 and other dielectric constants are determined with reference to it. Other properties of interest in a dielectric are dielectric strength, a measure of the maximum voltage it can sustain without significant conduction, and the degree to which it is free from power losses.
Dielectric strength is a measure of the electrical strength of a material as an insulator, Dielectric strength is defined as the maximum voltage required to produce a dielectric breakdown through the material and is expressed as Volts per unit thickness. The higher the dielectric strength of a material the better its quality as an insulator.
The dielectric strength of an insulator is the critical electric field beyond which the molecules in the dielectric stretch so much that one or more electrons are ripped free from the molecules. When this happens, the free electrons crash into other stretched molecules, knocking other electrons free; the resulting chain reaction is a spark that bums a hole through the dielectric. If this happens to the dielectric in a capacitor, the capacitor is usually ruined.
Thus an extremely desirable characteristic of a dielectric is a high dielectric constant.
2. Description of Related Art
As is known and described e.g. in U.S. Pat. No. 5.790,367 the quality of multilayer capacitors is determined by the chemical composition of their dielectrics and of the electrodes.
The use of barium titanium oxide otherwise known as barium titanate has been described in the prior art including in the above mentioned patent and in many other patents such as U.S. Pat. Nos. 4,219,866; 3,987,347 to mention but a few.
Similarly the use of alumina as a possible component in a dielectric composition is also mentioned in the prior art e.g. in U.S. Pat. No. 4,670,815.
Nevertheless, none of said patents teach nor suggest the combination of a major proportion of alumina and a minor proportion of barium titanium oxide together with other components to form a superior capacitor having a surprisingly high dielectric constant as well as excellent structural and electrical stability.